Microprocessor speeds have been increasing at a rapid pace. The present invention deals with that class of microprocessor which operates with different speed modes, including at least a high and low speed mode. With each increase in speed, there is a tendency for further heating of the device, as well as increased power consumption. Overheating of a chip leads to thermal instabilities and later to failure of transistors and of the entire chip. In a compact computer system, such as a laptop computer, there is a dual objective of prolonging battery life and restricting the temperature of the CPU to a tolerable level.
There are two basic temperature control schemes, a closed loop system and an open loop system. The closed loop system requires a temperature sensor attached to the point of temperature criticality of the microprocessor and an analog-to-digital converter to transfer the temperature measurement into a signal which can be used by the system hardware or software to determine whether or not the system speed should be reduced. This approach is very accurate but is more expensive than the open loop system. It also increases the complexity of the system design.
In most applications, an open loop temperature control system is used. Such a system predicts heat accumulation and heat dissipation as functions of both time and operating frequency. The system provides temperature control based upon a selected model. An example of this predictive approach is found in U.S. Pat. No. 4,425,624 (Plenche), where various known currents and resistances are used to model the thermal behavior of an electronic device to simulate temperature rise as a function of time.
If an open loop approach is applied to a variable or multiple speed microprocessor, the system is forced to switch speeds from a high frequency to a low frequency after running at a high frequency for a certain period of time. This technique, known as throttling, assumes the temperature changes are linearly proportional to the amount of time of operation of a microprocessor at a particular frequency.
The real situation is that the temperature versus time relationship is nonlinear and generally follows a flattening exponential curve. A second drawback is that the predictive approach usually counts low speed time due to throttling but ignores low speed time induced by system idleness. When a system idle condition occurs, the usual procedure is to reset the counting of high speed time or to stop counting. As a result, heat dissipation is either underestimated or overestimated.
An object of the invention is to provide an open loop temperature control system which more accurately emulates the temperature versus time relationship and provides microprocessor cooling accordingly.